The Planning Group for the coordination of acoustic surveys off Northwest Africa was held in Dakar, Senegal, 28-29 October 2003.
The general objective of the Planning Group is to plan the coordination of acoustic surveys in the region including intercalibration of research vessels and to act as a forum for discussion on issues important to acoustic surveys such as standardization of methods, acoustic research and training.
The meeting was organized by FAO in cooperation with the Oceanographic Research Center of Dakar-Thiaroye (CRODT) as one of the activities under Project GCP/INT/730/NOR “International Cooperation with the Nansen Programme: Fisheries Management and Marine Environment.”
A total of ten scientists from Gambia, Morocco, Mauritania, Norway, Senegal and FAO participated in the meeting. The list of participants is provided in Appendix A. Mr Abdoulaye Sarre (CRODT, Senegal) was elected Chairperson of the meeting for a two-year period. The Agenda for the meeting is attached as Appendix B.
The terms of reference were to:
In 2003 one acoustic survey was carried out. From 12 to 27 March 2003, R/V IT AFDÈME carried out a cold season acoustic survey on the continental shelf of Senegal. The southern zone was covered between 14 and 21 March and the northern zone between 21 and 23 March.
This survey forms part of the fish stock assessment programme of the EEZ of Senegal. The aim is to evaluate and map, by acoustic survey, the distribution of coastal pelagic fish on the Senegalese continental shelf.
The main aims are to:
Estimate the abundance of the fish populations of the Senegalese zone during the period when the thermal front is furthest to the north, the target species being: round sardinella (Sardinella aurita), flat sardinella (Sardinella maderensis), horse mackerel(Trachurus trecae) and(Decapterus rhonchus), and chub mackerel (Scomber japonicus).
Study the distribution of the populations in relation to environmental conditions and complete studies on the migration of the species related to the movement of the thermal front.
The main equipment used are an EK 60 echo-sounder, a BI 500 integrator and a 38 kHz ES388 transducer. In all, 26 radials parallel to the degrees of latitude and spaced at 5 nautical mile intervals were carried out on the southern coast (10 nautical mile spacings were used off the coast of Gambia), in the northern zone, 20 zigzagging radials containing parallels to the degrees of latitude spaced at 5 nautical miles and “inter-radials” which were almost perpendicular to the isobaths were carried out. Lastly, at the end of the survey, 6 radials that were almost perpendicular to the isobaths around St. Louis.
The results obtained show that on the whole there is a strong presence of sardinella during this season. Taking all species into consideration, of a global biomass estimated at around 1014000 tonnes, flat sardinella represent 30 percent and round sardinella 14 percent. The latter species is only present on the northern coast, with traces being recorded in the south during this period.
Horse mackerel are present along the entire coast and predominantly on the “Petite côte”, whereas chub mackerel tend to be more localized around Casamance during this season.
It should be noted that, with regards to pelagic fish, improvements need to be made in efficiency during the acoustic surveys.
In 2003 two acoustic surveys were carried out with R/V AL-AWAM.
i. The pelagic stock assessment survey (10–25 March 2003)
The biomass of the main target species is estimated at around 400 000 tonnes. It emerges that the greatest biomass relates to sardine (Sardina pilchardus) which represents around 77 percent of the total estimated biomass. Surface temperatures were lower than the seasonal average. An enlargement of the sardine distribution area was observed south of the Mauritania zone. This was confirmed by the catch, during the same period, of sardine in Senegal (Samb, B. Pers.com.; Sarre A., Survey Report ID 02–03).
During this survey, 27 trawls were carried out of which eight were monospecific (more than 90 percent of a single species).
The average surface temperature was 16.9°C with a maximum of 19.7°C, a minimum of 15.4°C and a typical difference of 0.78°C. The variation in surface temperature in relation to the seasonal average was -1.5°C.
During this survey the weakest species identification variation coefficient (11 percent) was for anchovy in the southern coastal stratum. This survey highlighted a strong recruitment in the main pelagic species: sardine, sardinella and anchovy. Anchovy being distributed along the entire coast of the Mauritanian EEZ.
ii. Methodology survey (1–6 June 2003)
This survey highlighted a strong recruitment in the main pelagic species: sardine, sardinella and anchovy. Anchovy and sardine are distributed along the entire coast of the study zone (CapeBlanc). The total biomass of sardine was estimated at 971 730 tonnes, sardinella 78000tonnes (99 percent round and 1 percent flat) and anchovy 318 200 tonnes.
The horse mackerel biomass was estimated at 76 410 tonnes (75 percent Cunene horse mackerel and 25 percent European horse mackerel) while a considerable biomass of chub mackerel (110520 tonnes) was found in the zone.
Six control trawls were carried out during this survey and 295 nautical miles were covered.
The climatic conditions were normal in relation to the seasonal average (cold season) in the zone. They are characterized by a very low water temperature and an intense upwelling.
After two surveys carried out in 2002, four methodology surveys were foreseen for 2003 shown in one summary table provided in Appendix C (Section C).
The AMA 01–03 survey carried out in February coincided with the regional acoustic training course organized by INRH for the scientists of the research institutes of West Africa. Practical calibration and acoustic survey exercises were carried out on board.
The goal of the AMA 02–03 survey in July was to calibrate the sonar equipment of the vessel (sounder, sonar and ADCP) before it was put into dry dock.
The AMA 03-03 survey carried out in August was intended to measure the noise level, as well as to compare the data collected by the vertical and horizontal soundings.
The AMA 04-03 survey, scheduled for December, will coincide with the arrival of R/VDR.FRIDTJOF NANSEN. This mini survey between Cap Juby and Cap Ghir will allow, amongst other things, an inter-calibration to be carried out between the two vessels.
Two acoustic surveys were carrried out by the R/V DR. FRIDTJOF NANSEN off Northwest Africa, one in October-December 2002 and one in May-July 2003.
General survey strategy
No major problems were noted for the surveys in 2002 and 2003 by the participants.
Table
1
Estimated
abundance of the main pelagic species surveyed during the
survey in October – December 2002
| Species | Latin name | Total estimate (tonnes) |
| Sardine | Sardina pilchardus | 6 367 018 |
| Round sardinella | Sardinella aurita | 1 434 038 |
| Flat sardinella | Sardinella maderensis | 988 005 |
| Anchovy | Engraulis encrasicolus | 35 693 |
| Atlantic horse mackerel | Trachurus trachurus | 280 941 |
| Cunene horse mackerel | Trachurus trecae | 350 119 |
| False scad | Caranx rhonchus | - |
| Chub mackerel | Scomber japonicus | 291 860 |
Table
2
Estimated
abundance of the main pelagic species surveyed during the
survey in May – July 2003
| Species | Latin name | Total estimate (tonnes) |
| Sardine | Sardina pilchardus | 7 621 555 |
| Round sardinella | Sardinella aurita | 1 168 511 |
| Flat sardinella | Sardinella maderensis | 1 133 123 |
| Anchovy | Engraulis encrasicolus | 213 323 |
| Atlantic horse mackerel | Trachurus trachurus | 362 320 |
| Cunene horse mackerel | Trachurus trecae | 792 438 |
| False scad | Caranx rhonchus | 65 503 |
| Chub mackerel | Scomber japonicus | 373 956 |
Other results and local estimates can be found in the respective cruise reports.
Suggested improvements
The improvements will not take the focus away from the main objectives during the survey, which are to “map the distribution and estimate the biomass of the main small pelagic fish species using hydroacoustic methods”. Points 1 and 2 will be introduced during this years survey in October-December. With regards to Point 3, participants from the region specifically requested training in identification of acoustic targets and use of survey software. The local participant will produce a document on typical acoustic targets and school shapes during the upcoming Nansen survey and the Norwegian participants of the survey will give training in the use of the software onboard R/V DR. FRIDTJOF NANSEN. Point 4 should be discussed further and aimed to be implemented during the October-December survey 2004.
The results presented for each vessel are summarized in the tables in Appendix C.
All countries produced reports from their acoustic surveys. The survey reports in the region followed the format of the R/V DR. FRIDTJOF NANSEN cruise reports (see format reports Appendix D) and it was agreed that this could be used as a standard format in future surveys.
No major changes were suggested to the report format. It was suggested that the narrative of the report be extended to include all major events during the survey.
Standard format (from NANSEN reports):
TABLE OF CONTENTS
CHAPTER 1
INTRODUCTION
1.1 Survey
objectives
1.2
Participation
1.3
Narrative
Figure: The
cruise track and the stations
CHAPTER 2
METHODS
2.1
Environmental Data
2.2
Biological Sampling
2.3
Acoustic Sampling
CHAPTER 3 SURVEY
RESULTS
3.1 Weather conditions and hydrography
Figures:
Distribution of surface temperature, wind etc.
Figures: Vertical
temperature, salinity and oxygen profiles from standard hydrographical
transects
3.2 Biomass estimates and Distribution of main species
of pelagic fish
Figures: Distribution Maps
Tables: Biomass estimates of main species per region
3.3 Other results, e.g. biological
data, length, weight results, maturity, condition factor feeding studies, etc.
Tables and Figures as needed
CHAPTER 4 CONCLUDING REMARKS
ANNEX I Records of fishing stations
ANNEX II Instruments and fishing gear used
ANNEX III Calibration results
ANNEX IV Biomass and number by length
ANNEX V Pooled length distribution of
the main species
ANNEX VI Tables:
age distribution of the main species (if possible)
The meeting was informed about the activities carried out in 2002 and 2003 related to age determination of sardinella and sardine.
For sardinella, a first otolith exchange was carried out during the period May-November 2002 followed by a Workshop on the age estimation of sardinella (Sardinella aurita and Sardinella maderensis) held in Dakar, Senegal, from 4 to 7 February 2003. The main objective of the Workshop was to discuss the age-reading methodologies for the ageing of sardinella using otoliths and to establish age-reading criteria. At the Workshop, a set of preliminary criteria for the interpretation of sardinella otoliths was established and sampling programmes were initiated in Senegal and Mauritania to provide monthly samples of sardinella otoliths to be analysed in a second exchange. The second exchange was initiated in October 2003 with one set of otoliths being sent to Russia and one to Spain. The exchange will be followed by a Workshop to be held in Tenerife, Spain, in the first week of December 2003. The main objective of that Workshop will be to agree upon age reading criteria for sardinella and to establish an age-length key. The exchanges and workshops include participants from Gambia, Mauritania, Morocco, Netherlands, Russia, Senegal, Spain and FAO.
For sardine, an otolith exchange was initiated in June 2003. Those otoliths have so far been read by Morocco, Russia and Spain and should be on their way to Mauritania. The objective of the exchange is to investigate whether the readers adhere to the agreed criteria for the age determination of sardine, and to investigate the need for a new otolith exchange.
It is envisaged that an otolith exchange of horse mackerel will be carried out in 2004. It was therefore proposed that horse mackerel otoliths be collected during the October-December 2003 survey with R/V DR. FRIDTJOF NANSEN.
The planning meeting was informed that the Guidelines for Acoustic Surveys that were initiated in 1998 and discussed at the first Planning Group for coordination of surveys in Dakar, Senegal, in October 2002 were close to finalization. It was recommended that the final version should be circulated to the Planning Group members before printing.
R/V ITAF DÈME
During the acoustic surveys in Senegal, fishing operations were carried out by a pelagic trawl and often also by a demersal trawl as well, used as a pelagic trawl by keeping it on the surface with the aid of large floats. This was done in relation to the size of the echoes measured by the echosounder. At each fishing station, a sample was taken to determine the composition, weight, number per species and the length frequency of the target species. The length frequencies were measured to the half-centimetre below. In relation to the biological study which is being undertaken and for which monthly samples are collected at the landing sites, supplementary sardinella samples are collected throughout the experimental surveys and kept onboard to be processed later on land.
R/V AL-AWAM
During each trawl a random sample of the total catch is taken, the contents of each bucket are sorted, weighed and measured. The weight of each species is noted as well as all the details of the trawl in question (date, fishing zone, amount of fishing time, etc.).
- If the catch is large:
Two buckets (each weighing around 40 kg) of the total catch are taken, and once sorted, the species are identified. A sample of 100 individuals per species is measured. Biological analysis is carried out on the target species at a rate of 20 individuals per species.
- If the catch is less sizeable:
The whole catch is subject to identification and weighing.
Catch composition
Large species are all sorted. They are then weighed and counted. For the rest of the catch, a sample is taken of one in five trawls, depending on the type of catch. The composition is then estimated based on the total sample catch.
Length measurement
The following species of small pelagic are systematically measured to the nearest centimetre below (total length): Sardinella maderensis, Sardinella aurita, Trachurus trecae, Trachurus trachurus, Decapterus rhonchus, Scomber japonicus, Sardina pilchardus, Engraulis encrasicolus.
Biological analyses
A sample of sardinella, and occasionally other small pelagic species, is used to determine sexual maturity, fat content and repletion. Analyses are carried out in the onboard laboratories which are equipped with scales capable of weighing up to 10 kg and a lab bench.
Supplementary analyses (ageing, a study of reproduction) are carried out on the samples that are brought back to the laboratory.
R/V AL AMIR MOULAY ABDALLAH
Biological sampling has been the subject of particular attention. The fishing operation is followed by net sonde and the semi-pelagic trawl is hauled as soon as the catch is estimated to be sufficient, so that the deck does not become obstructed.
The species are all sorted. Proportions are extrapolated. They are then weighed and counted, the target species are systematically sampled. Their ratios to the total catch and the length frequencies are measured. The biologist in charge measures the biological parameters of each target species and takes sub-samples for further measurements in the biology laboratory (ageing, genetics, etc.).
The total length of the target species is measured to the nearest centimetre below for chub mackerel and horse mackerel, and to the half centimetre below for sardine and anchovy.
One of the major constraints in estimating pelagic stocks by the method of echo-integration is that the allocation of retro-differed energy in its different specific elements creates a bias and major source of errors. For the two surveys carried out in Senegal, the mono-specific shoals can be illustrated by typical echograms: they are the flat sardinella and the horse mackerel (see Appendix H).
Although it is difficult to extract clear species identification indices, some rough indications give an idea of the nature of what is detected:
Flat sardinella are more often found in coastal zones, whereas horse mackerel are found in deeper waters.
Mauritania presented the type of echogram classification used in the R/V AL-AWAM surveys (Appendix H). This classification which divides the echotraces into four categories, represents a step towards characterising the echotypes found in the Mauritanian EEZ:
D1 - Energy attributed to sardinella, sardine and anchovy corresponding to dense shoals near the surface, found close to the coast or out to sea.
D2 - Energy attributed to horse mackerel and chub mackerel corresponding to dispersed shoals or found at depths near the bottom.
D3 - Energy attributed to other pelagic species showing different characteristics to D1 and D2.
D4 - Energy attributed to dense layers of plankton and to the mesopelagic zone.
It is recommended that the different countries:
It should be underlined that this task must involve the team leaders from each country in order to obtain the desired results.
An overview of the surveys planned for 2004 for each vessel in the subregion, is given in Appendix E. A detailed programme of the acoustic surveys in particular is given in AppendixF.
R/V ITAF DÈME
Two acoustic surveys are planned for 2004, aimed at estimating the abundance of coastal small pelagics along the Senegal-Gambian continental shelf during the warm and cold seasons. The cold season survey will be carried out in conjunction with R/V DR. FRIDTJOF NANSEN in order to perform an intercalibration between the two vessels.
R/V AL-AWAM
Four acoustic surveys are planned. Two will be carried out to estimate the abundance of coastal small pelagics in the Mauritanian EEZ during the cold season and the change between the cold and warm seasons. The second survey will be carried out in conjunction with R/VDR.FRIDTJOF NANSEN in order to perform an intercalibration between the two vessels.
The other surveys are methodological. Several aspects will be covered: TS measurement, pelagic trawling, comparison between night and day, species identification, echostructure and echotype of the target species.
R/V AL AMIR MOULAY ABDALLAH
One proposal to use R/V AL AMIR MOULAY ABDALLAHfor the surveys, is given in Appendix E. It is similar to that proposed in 2003. The model involves seven surveys at sea, two oceanographic surveys lasting 30 days each, four pelagic surveys of ten days each (two in the North Atlantic region and two in the Mediterranean), one demersal survey on the rocky bottoms of 25 days, and one inter-calibration with R/V DR. FRIDTJOF NANSEN.
The zones surveyed are additional to those covered by R/V DR. FRIDTJOF NANSEN and the survey periods have been chosen in such a way as to allow intra- and inter-annual comparison.
R/V Dr. Fridtjof Nansen
The complete survey plan for the R/V DR. FRIDTJOF NANSEN in 2004 was not yet available at the time of the meeting, but a draft plan is expected to be available shortly. The meeting was informed that NORAD had agreed to continue the funding for another two-year period, but that this period should be used to investigate the possibilities of finding alternative sources of funding for the Programme. Consequently, it may be that only one acoustic survey will be undertaken off Northwest Africa during 2004. However given the uncertainty of this, it was decided to plan for the two surveys and to retain the same objectives as for the 2003 surveys, with the additional objective of carrying out a parallel survey.
Calibration will be carried out in Morocco in December 2003. In 2004 no calibration exercise is envisaged until the parallel survey.
It was agreed that a parallel survey between the three local research vessels and R/VDR.FRIDTJOF NANSEN should be carried out in October-December 2004, and a preliminary plan was established. Detailed plans are still to be elaborated.
The suggested plan below must be discussed before the parallel survey. The settings for the acoustic equipment, the data processing and comparison by team leaders on board R/VDr.Fridtjof Nansen must also be discussed.
28 October:
- R/V Dr. Fridtjof Nansen and R/V Itaf Dème depart Dakar.
28 October – 7 November 2004:
- Survey of the Senegalese and Gambian shelves by R/V Dr. Fridtjof Nansen and R/VItaf Dème.
8 November:
- Saint-Louis offshore. Transfer data from R/V Itaf Dème to R/V Dr. Fridtjof Nansen for analysis and comparison on board R/V Dr. Fridtjof Nansen.
- R/V Itaf Dème returns to Dakar.
- R/V Al-Awam arrives in Saint-Louis offshore.
9–19 November:
- R/V Dr. Fridtjof Nansen and R/V Al-Awam cover the Mauritanian shelf.
20 November:
- Cap Blanc offshore. Transfer data from R/V Al-Awam to R/V Dr. Fridtjof Nansen for analysis and comparison on board R/V Dr. Fridtjof Nansen.
- R/V Al-Awam returns to Nouakchott.
- R/V Al Amir Moulay Abdallaharrives in Cap Blanc offshore.
21 November –16 December:
- R/V Dr. Fridtjof Nansen and R/V Al Amir Moulay Abdallah cover the Moroccan shelf.
17–20 December:
- Agadir. Transfer data from R/V AL AMIR MOULAY ABDALLAH to R/V DR. FRIDTJOF NANSEN for comparison on board R/V DR. FRIDTJOF NANSEN.
The R/V Dr. Fridtjof Nansensurveys in the Northwest Africa region have both estimated the abundance of coastal small pelagic stocks and mapped their distribution. This series of data represents an abundance index in relation to the different sources of bias and in particular towards the reflection index. In order to improve the estimates, particular effort should be aimed at the estimation of the species specific target strength (TS).
Following the recommendation on TS measurements at the Workshop to review the acoustic surveys off the Northwest African coast (Bergen, Norway, 28 September -2 October 1998) and taking into consideration the varied nature of the fisheries and their behaviour within the pelagic ecosystem of the Northern CECAF region, it is necessary to undertake certain measures for the main species, so as to establish a reliable TS equation for each species.
Thus a project has been designed by the participants from each country for TS measurement meetings to be held throughout 2004, proceeded by a theory course on the methodology of TS measurement by Norwegian experts. Mauritania has been chosen to oversee the proceedings. The project is given in Appendix G.
During the discussion about data collection a question came up about how the institutions stored their scientific raw data after the surveys. Collection of these data are expensive and time consuming, and proper storage with back ups are vital since lost data will never be retrievable. Several institutions did not have a strategy for storage of data and had in the past experienced that scientific information had been lost. It was noted that the institutions did not have a strategy for storage and quality control of research data and that they expressed interest in trying to implement a common data storage strategy and possibly a regional research database. It was suggested by the group that a regional data storage strategy should be discussed further by the next steering committee meeting.
Gambia expressed their wish for the R/V Dr. Fridtjof Nansen to call at Banjul during acoustic surveys in Senegal-Gambia and also to participate in Senegalese national surveys.
It was noted during the meeting that those countries owning vessels, each carried out acoustic surveys in 2003 and that real progress has been made in the planning and execution process and in data processing with regard to acoustic evaluation. It appeared that at the sub-regional level there had been real capacity strengthening in collaboration with the Nansen Programme.
Nevertheless, agreement still has to be reached at the coordination level of national surveys, on the study of the noise level and on mastering pelagic trawling techniques with a view to correctly sampling coastal pelagic fish.
The Working Group recommended focusing on the following in 2004:
To characterize the echotypes of the target species during the October-December 2003 survey on board the R/V Dr. Fridtjof Nansen. To this end a technical report should be written including the echograms. The mission leaders in the respective countries will be responsible for this.
The need to carry out parallel surveys between R/V Dr. Fridtjof Nansen and the national vessels in October-December 2004 was agreed upon and a plan drawn up.
Morocco expressed an interest in carrying out an intercalibration between R/V Dr. Fridtjof Nansen and R/V Al Amir Moulay Abdallah in December 2003, the group recommended that this should be considered and discussed with the Norwegian cruise leader.
The need to carry out methodological studies on the TS reflection index was underlined and a project aimed at carrying out this study was drawn up by all the participants. This proposal will be put to the Steering Committee of the Nansen Programme.
The Group recommended that Gambia contact Senegal so that it might be allowed to participate in the national surveys of Senegal.
The Working Group suggests to the Steering Committee that discussions be held in the near future on the possibility of working out a common strategy to safeguard the pelagic survey data of the sub-region.
